The number of charges of constituent particles of an aerosol has generally a distribution. Neutralizing techniques which make the average of the charge distribution nearly zero are widely used as important techniques in the field of particle size distribution measurements of aerosol particles by electric mobility classification, Electric mobility distribution measurements by neutralizing techniques have been discussed in detail (refer to Non-Patent Document 1). Products utilizing neutralizing techniques have been made commercially available by a plurality of manufacturers including USA TSI Incorporated. These measurement apparatus have been used widely for measuring a particle size distribution in a manufacture process using fine particles, and a particle size distribution of fine particles in atmospheric aerosol or in a car exhaust gas.
Although most of constituent particles of aerosol in a neutralized charge distribution are not charged, there are some particles having positive or negative monovalent or multivalent charges. The number of particles at each valence is almost equal for positive and negative particles. In a graph having as an abscissa a charge valence and as an ordinate an existence frequency of particles at each charge valence, an existence frequency distribution is in positive/negative symmetry having zero as a most frequent value. This charge distribution state is called a neutralized state. A distribution of the number of charged particles and a distribution of the ratio between charged and uncharged particles are already known for each particle diameter in the neutralized state. Therefore, a particle size distribution of all particles including uncharged particles can be converted from a particle size distribution of charged particles measured by an electric mobility method, and can be calculated precisely.
An apparatus utilizing radioactive substance is used most frequently to neutralize aerosol particles. The neutralizer of this type is described in detail, for example, in Non-Patent Document 1, and an example of the structure is shown in FIG. 8. In this neutralizer 50, high energy particles radiated from radioactive substance 51 collide with gas molecules to generate a number of ions, the numbers of whose positive and negative ions are nearly equal. Bipolar ions generated in this manner attach to floating particles during a Brownian motion so that a charged particle quantity of each particle changes. In the state that nearly the same numbers of positive and negative ions exist, an attachment probability of an ion to a charged particle having a polarity opposite to that of the ion is larger than an attachment probability of an ion to a charged particle having the same polarity to that of the ion. Therefore, this attachment reaction between particles and positive and negative ions makes most of particles uncharged. However, some particles are charged positive or negative monovalent, and a smaller number of particles are charged positive or negative multivalent. As a whole, the above-described neutralized state is achieved.
To generate bipolar ions so as to neutralize aerosol, electrical discharges may be utilized. For example, at the same time when positive ions are generated through positive DC corona discharge, negative ions are generated through negative DC corona discharge to mix these ions and obtain negative and positive ions whose numbers are almost equal. This neutralizer has an ion generating field separated from a particle charge neutralizing field. This separation is necessary for preventing a loss of particles in the DC corona discharge field (refer to Non-Patent Documents 2 and 3). There is discussion on application of a bipolar ion generator apparatus utilizing an AC corona generator apparatus to neutralization of aerosol particles. With this neutralizing method, however, the bipolar ion generating field is separated from the particle charge neutralizing field, too (refer to Patent Document 1).
A charge neutralizer utilizing creepage barrier discharge and an AC power source (refer to Patent Document 2) has characteristics that (1) a relatively high frequency is required to obtain a high ion concentration, (2) a high ozone concentration, and (3) no bias is necessary for the control of ion balance.
As the neutralizing techniques through generation of bipolar ions, there is another technique for generating positive ions and photoelectrons utilizing photoelectron emission by ultraviolet irradiation (refer to Patent Document 3).
This method, however, depends on a principle that the numbers of positive/negative ions are adjusted by generating a DC electric field in the neutralizer. Therefore, charged particles are transported to the neutralizer wall by the electric field in the neutralizer and lost. Since electric mobility measurements are effective only for charged particles, this neutralizing method in combination with the electric mobility measurement is not suitable for practical use.
There are a number of other proposals and measures relative to the techniques of adjusting a charge distribution of aerosol particles. These techniques do not aim at neutralization, but most of these techniques aim to change particles in an uncharged state to particles in a charged state, by utilizing positive or negative single polarity ions.
Charging techniques and transport controlling techniques aiming to facilitate transport control of particles in space through particle charging, are utilized for improvement of a productivity efficiency in a manufacture process using particles as material particles (refer to Patent Document 4), for control of toner particles in a copy machine (refer to Patent Document 5), for removal of particles in air of electric dust collection (refer to Patent Document 6), and for increase of a measurement sensitivity of a particle measuring apparatus having a sensitivity only to charged particles (refer to Patent Document 7).
Among these charging techniques, monopolar ions generated by DC discharge are used (refer to Patent Document 8), or only single polarity components are extracted from bipolar ions generated from radioactive substance (refer to Patent Document 9). However, with any of these approaches aiming to charge uncharged particles, a charge distribution of particles is shifted from zero to either positive or negative, and a neutralized state of the charge distribution cannot be realized. Further, since a number of multivalent charged particles are generated, there arises a problem of sensitivity crossing that a large particle having multivalent charges and fine particles having a monovalent charges are measured as having the same electric mobility, in the particle distribution measurements by the electric mobility method. It is therefore difficult to apply such particle charging techniques directly to the neutralizing techniques aiming at particle diameter measurements by the electric mobility method.    Patent Document 1: Japanese Patent Publication No. 3393270    Patent Document 2: Japanese Patent Unexamined Publication No. 2005-106670    Patent Document 3: Japanese Patent Publication No. 2670942    Patent Document 4: Japanese Patent Unexamined Publication No. 2002-190258    Patent Document 5: Japanese Patent Unexamined Publication No. 2000-187369    Patent Document 6: Japanese Patent Unexamined Publication No. SHO52-99480    Patent Document 7: Japanese Patent International Publication No. 2000-504111    Patent Document 8: Japanese Patent Publication No. SHO-62-19033    Patent Document 9: Japanese Patent Publication No. HEI-24357
Non-Patent Document 1: Knutson, E. O. (1976), Extended electric mobility method for measuring aerosol particle size and concentration, Fine Particles, Aerosol Generation, Measurement, Sampling, and Analysis. B. Y. H. Liu. New York, N.Y., Academic press: 740-762.
Non-Patent Document 2: Adaci, M. et al. (1993), “Aerosol charge neutralization by a corona ionizer.” Aerosol Sci. Technol. 18:48-58.
Non-Patent Document 3: Wiedensohler, A. (1988). “An approximation of the bipolar charge distribution for particles in the submicron size range.” J. Aerosol Sci. vol. 19, 3:387-389.